Stenosis of either the portal or splenic vein increases splenic afferent nerve activity (SANA) which, through the splenorenal reflex, reduces renal blood flow. Since these maneuvers not only raise splenic venous pressure, but also reduce splenic venous outflow, the question remained as to whether it is increased intrasplenic post-capillary pressure and/or reduced intrasplenic blood flow which stimulates SANA. In anesthetized rats, we measured the changes in SANA in response to partial occlusion of either the splenic artery or vein. Splenic venous and arterial pressures and flows were simultaneously monitored. Splenic vein occlusion increased splenic venous pressure (9.5±0.5 to 22.9±0.8 mmHg, n=6), reduced splenic arterial blood flow (1.7±0.1 to 0.9±0.1 ml/min, n=6) and splenic venous blood flow (1.3±0.1 to 0.6±0.1 ml/min, n=6), and increased SANA (1.7±0.4 to 2.2±0.5 spikes/sec, n=6). During splenic artery occlusion, we matched the reduction in either splenic arterial blood flow (1.7±0.1 to 0.7±0.05, n=6) or splenic venous blood flow (1.2±0.1 to 0.5±0.04, n=5) with that seen during splenic vein occlusion. In neither case was there any change in either splenic venous pressure (-0.4±0.9mmHg, n=6 and +0.1±0.3mmHg, n=5) or SANA (-0.11±0.15 spikes/sec, n=6 and -0.05±0.08 spikes/sec, n=5) respectively. Furthermore, there was a linear relationship between SANA and splenic venous pressure (r=0.619, p=0.008, n=17). There was no such relationship with splenic venous (R=0.371, p=0.236, n=12) or arterial (R=0.275, p=0.413, n=11) blood flow. We conclude that it is splenic venous pressure, not flow, which stimulates splenic afferent nerve activity, and activates the splenorenal reflex in portal and splenic venous hypertension.